U.S. Pat. No. 5,178,215 serves as a starting point for the departure made by the present invention. The disclosure of U.S. Pat. No. 5,178,215 is incorporated herein by reference and includes a general discussion of an existing rotary blowout preventer which is fluid actuated to grip a drill pipe or kelly, and the controlled circulation of a fluid to lubricate and cool bearings and seals, and to filter particulate matter.
These existing rotary blowout preventers have an annulus between an outer housing and a rotary housing. Such systems use rather large bearings which require a rather large clearance. Such an arrangement has positive effects but also results in xe2x80x9cwobblingxe2x80x9d between the rotary housing and the outer housing. The wobbling creates heat, xe2x80x9cnibblesxe2x80x9d the seals, etc. A fluid is introduced into and circulates through the annulus between the outer housing and the rotary housing to cool the seal assemblies, the bearings and to counteract heat generated by contact between the seals and the rotary housing (wellhead fluid temperatures may normally be about 200xc2x0 F., and during rotation, without cooling, the temperature would readily increase to about 350xc2x0 F. and destroy a seal in a relatively short time). The circulated fluid also removes foreign particulate matter from the system. Pumps are used to maintain a fluid pressure in the annulus at a selected pressure differential above the well bore pressure.
The bearings in these rotary blowout preventers may normally operate at a temperature of about 250xc2x0 F. Such bearings are subjected to a significant thrust load, e.g. 2,000 lbs.-force, due in part to an upward force created by well bore pressures and placed upon a packer assembly and a sleeve in the rotary housing. Such a thrust load will generate significant heat in a bearing rotating at, for example, 200 rpm. Heat, and heat over time, are important factors which may lead to bearing failure. For example, bearings may immediately fail if they reach temperatures of about 550xc2x0 F. Even at temperatures of 250xc2x0 F. a bearing may fail after a significant period of use, for example, twenty days of rotation at 200 rpm when subjected to a significant thrust load.
Such existing rotary blowout preventers are very functional at wellhead pressures up to 2000 psi. However, for reasons discussed herein, there are added challenges when wellhead pressures are in the range of, for example, 2500 psi to 5000 psi.
For example, as suggested, the continued and trouble free operability of such rotary blowout preventers is dependent, in part, upon the life of the seals and bearings within the rotary blowout preventer. The seals have a xe2x80x9cpressure/velocityxe2x80x9d or xe2x80x9cpvxe2x80x9d rating which may be used to predict the relative life of a seal given the pressure and velocity conditions to be borne by a seal. When considering xe2x80x9cPVxe2x80x9d rating, it is significant to note that a linear relationship does not exist between the life of a seal and the increases in pressure or rotational velocity to which a seal will be subjected. Rather, the life of the seal decreases exponentially as the pressure or rotational velocity to which the seal is subjected is increased.
As such, when well bore pressures increase to ranges from 2500 psi to 5000 psi, the loads, the wear and the heat exerted on seals and bearings within a rotary blowout preventer pose a greater challenge to the operations and life of the seals and bearings. This must be considered in the context of the fact that well bore operations may be shut down for maintenance work when significant wear of seals or bearings, significant xe2x80x9cnibblingxe2x80x9d of seals, or seal/bearing failure occurs. Such shut downs can significantly affect the profitability of well bore operations.
This rotary blowout preventer has a first and a second pressurized fluid circuit. Each of the fluid circuits are defined into and out of a stationary body and between the stationary body, a rotating body, and two seals. The first fluid circuit is physically independent from the second fluid circuit although they share a seal interface. A fluid is introduced into the first fluid circuit at a pressure responsive to the well bore pressure. A fluid is introduced into the second fluid circuit at a pressure responsive to and lower than the pressure of the fluid in the first circuit. Adjustable orifices are connected to the outlet of the first and second fluid circuits to control such pressures within the circuits. Such pressures affect the wear rates of the seals. The system can therefore control the wear rate of one seal relative to another seal. A thrust bearing is added to share the load placed upon the upper bearings. The thrust bearing is connected between the top end of a packer sleeve and the stationary body.